Diamond is an allotrope of carbon which is metastable at ordinary pressures, having a large activation energy barrier which prevents conversion to graphite, which is the more stable allotrope at ordinary temperatures and pressures. In addition to its value as a precious gem, the uses of diamond include many industrial applications such as polishing, grinding, sawing and drilling, as windows in high pressure cells, in diamond microtone knives for biological and medical uses, as a radiation detector, for temperature measurement, as heat sinks, as wire drawing dyes, styli for phonographs and as hardness indenters. Thus numerous approaches have been utilized to attempt to synthesize diamond.
One approach known in the prior art is to utilize high pressure methods since at high pressures diamond, rather than graphite, is the thermodynamically stable form of carbon. However, heretofore, high pressure methods have met only with limited commercial success since only small diamond crystals have been made, which are suitable mainly for use as abrasives and in forming sintered preforms for use as wire drawing dyes or tool bits. Moreover, the product of high pressure diamond synthesis is often contaminated with impurities of more or less uncontrollable concentration and distribution, rendering such diamond unsuitable for a number of important technical applications.
There have been attempts to grow diamond under low pressure conditions which, at first impression, may seem to be against thermodynamic principles. However, upon crystallization at low pressures using free carbon atoms, the carbon atoms, during their fall from a state of higher free energy, may be made to pause (i.e. crystallize) at the level of diamond, instead of forming graphite. Therefore free carbon atoms with a higher free energy than that in diamond may be made to crystallize as diamond under suitable conditions, thus the metastability of diamond alone is not a deterrent factor of obtaining diamond at atmospheric or reduced pressure. Moreover, metastable phases, such as diamond, may be made to grow in the stability field of another phase, when nucleation and growth is facilitated by providing seeds of the required phase or a substrate which allows epitaxial overgrowth. Thus there are numerous diamond growth utilizing low pressure epitaxial crystallization of diamond. However, these gas phase synthesis techniques suffer from the problems of extremely low growth rates and/or the inevitable problem of interruption of growth due to formation of graphite. Once the graphite is formed, being the favored thermodynamic product at low pressure, it overtakes and inhibits further diamond growth. In order to maximize the time available for diamond growth before the appearance of graphite, the vapor pressure of the carbon bearing gas has been usually kept quite low, thus leading to very slow diamond deposition rates, typically about 0.1 micron/hour.
It has only recently been reported that atomic hydrogen is important to epitaxial diamond growth. Pate (Ph.D. thesis, Stanford University, 1984), elucidated the suggestion by Russian workers (Varnin, et al. Soviet Physics - Crystallography 22(4), pp. 513-515, 1977) that atomic hydrogen, adsorbed on the diamond epitaxial surface, acts to stabilize carbon sp.sup.3 bonding (diamond bonding) rather than sp.sup.2 bonding (graphitic bonding), thereby favorably altering the kinetics of diamond-bond-formation carbon atoms and the growing diamond surface. Without atomic hydrogen, or other means of achieving the desired effect of stabilizing or enhancing sp.sup.3 bond formation versus sp.sup.2 bond formation, diamond growth by low vapor deposition is relatively inefficient and undesirable graphite deposition occurs rapidly.
There are many processes known in the art in which diamond is synthesized under high pressure at which diamond is the thermodynamically stable form of carbon. Although there are many variations of this technique, a typical technique involves use of a suitable carbon solvent such as a transition metal alloy, and a carbon source which are compressed and heated in an apparatus capable of providing pressures of at least 60 kilobars at temperatures above 1500.degree. C. The carbon is dissolved, transported, and deposited as diamond crystals but the carbon transport rate is governed primarily by diffusion, and therefore is very low. Thus the growth rates are slow and long deposition times are required to grow large diamonds. Furthermore because of the high pressures and temperatures required, the apparatus is necessarily bulky, expensive. Furthermore, because of the relatively small active volume, high pressure deposition precludes effective use of mechanical techniques (such as stirring) which might improve growth rates and product quality.
It is therefore an object of the present invention to provide a method for low pressure vapor phase metastable diamond synthesis whereby large mechanically stable diamond masses may be produced.
It is another object of the present invention to provide a method for consolidating diamond particles, or diamond-coated particles, into a large mechanically stable diamond mass substantially devoid of interstitial spaces.
It is yet another object of the present invention to provide a method for low pressure vapor phase metastable diamond synthesis wherein the free carbon atoms are deposited in the presence of atomic hydrogen to improve deposition of diamond over the formation of graphite.
It is yet another object of the present invention to provide a method for low pressure vapor phase metastable diamond synthesis in the presence of atomic hydrogen, whereby the atomic hydrogen is provided by dissociation of molecular hydrogen upon a catalytic matrix.
These and other objects of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments and from the appended claims.